A distributed DC power supply including a solar cell, a fuel cell, or the like includes a power conditioner configured to convert a frequency and a voltage to AC power adapted to a commercial power system in order for use in interconnection with the commercial power system.
The power conditioner includes a DC/DC converter configured to adjust DC power generated by the solar cell, the fuel cell, or the like to DC power having a predetermined voltage value, a DC/AC inverter configured to convert the DC power outputted from the DC/DC converter to AC power, an LC filter configured to remove a high frequency component from an output waveform of the DC/AC inverter, and the like.
In a case where a shunt fault occurs on a distribution line used by the power conditioner in grid connected operation, connected to the solar cell, the fuel cell, or the like, or power transmission from a substation to the distribution line stops due to planned power outage or the like and islanding operation is established, in order to prevent influence on operation of a sectionalizing switch and secure safety during maintenance of the distribution line and the like, the power conditioner includes a control unit configured to open a grid interconnection relay to separate the distributed power supply from the distribution line.
When the control unit of the power conditioner subsequently closes a stand-alone power system relay, the distributed power supply supplies AC power to a stand-alone power system separated from the commercial power system or to an independent stand-alone power system not interconnected with the commercial power system.
The control unit of the power conditioner includes a current control block configured to control the DC/AC inverter so as to output an AC current in synchronization with a phase of the commercial power system upon grid interconnection, and a voltage control block configured to control the DC/AC inverter so as to output an AC voltage at a predetermined level to the stand-alone power system upon power system separation.
The voltage at the predetermined level corresponds to a voltage for low-voltage customers prescribed in Electricity Business Act, Article 26 and the Ordinance for Enforcement of the Act, Article 44, and falls within 101±6 V with respect to a standard voltage 100 V and within 202±20 V with respect to a standard voltage 200 V.
Upon grid independent operation of supplying power to a stand-alone power system while separated from a commercial power system, it is necessary to detect beforehand whether or not a contact of a grid interconnection relay is normal in order to prevent reverse charge to the commercial power system and asynchronous input. If the contact of the grid interconnection relay is abnormal due to welding or the like, it is necessary to inhibit transition from grid connected operation to grid independent operation.
Patent Literature 1 discloses a grid interconnection device including a filter circuit configured to smooth AC power from an inverter circuit, an inverter circuit controller configured to control an operation state of the inverter circuit, a controller configured to control an interconnected state or a separated state of a grid interconnection relay, a current detector connected between the filter circuit and the grid interconnection relay and configured to detect a current flowing to the filter circuit, and an abnormality detector configured to detect an abnormality of the grid interconnection device in accordance with a control state of the grid interconnection relay and a detection result by the current detector while the inverter circuit controller controls to stop the inverter circuit.
The abnormality detector is configured to determine whether or not the contact of the grid interconnection relay is welded in accordance with whether or not a reactive current flows from the commercial power system to a capacitor of the filter circuit while the inverter circuit is stopped.
Patent Literature 2 proposes a grid interconnection device configured to detect whether or not a first grid interconnection relay or a second grid interconnection relay each included in a grid interconnection relay has welding before a DC/AC inverter circuit is interconnected with a commercial power system by detecting, by means of a photocoupler or the like, a potential difference between an input end of the first grid interconnection relay and an output end of the second grid interconnection relay, and a potential difference between an output end of the first grid interconnection relay and an input end of the second grid interconnection relay while the commercial power system normally operates, the inverter circuit is controlled to stop, and the grid interconnection relay is controlled to open.
The grid interconnection device is configured to execute detection in a similar manner by operating the inverter circuit upon power cut of the commercial power system.
Patent Literature 3 discloses a grid interconnection inverter device configured to check that both an interconnection switch and an inverter circuit are normal and then safely start operation interconnected with a commercial power system.
The grid interconnection inverter device includes the inverter circuit configured to convert DC power supplied from a DC power supply to AC power, an output voltage detector configured to detect an output voltage of the inverter circuit, the interconnection switch configured to establish interconnection between the inverter circuit and the commercial power system, a grid voltage detector configured to detect a voltage of the commercial power system, and a control circuit configured to control the inverter circuit and the interconnection switch.
The control circuit is configured to check that the interconnection switch is opened in accordance with a detection value of the output voltage detector and then start the inverter circuit, and control to close the interconnection switch if the detection value of the output voltage detector is substantially equal to a detection value of the grid voltage detector.
The grid interconnection inverter device is provided, between the inverter circuit and the interconnection switch, with a current limiting resistor configured to consume a current generated by a voltage difference between the inverter circuit and the commercial power system, and a resistor short-circuit switch configured to short-circuit the current limiting resistor. The grid interconnection inverter device is configured to control to close the resistor short-circuit switch at predetermined timing after the control circuit closes the interconnection switch.
Patent Literature 4 discloses a power conversion device configured to detect an abnormality of a switch disposed between a power converter and a grid independent operation terminal.
The power conversion device includes the power converter configured to convert power supplied from an external device to predetermined power, a first switch connected between a grid connected operation terminal connected to a power system and the power converter, a second switch connected between the grid independent operation terminal connected to a load and the power converter, a voltage detector configured to detect a voltage of the grid independent operation terminal, and a controller configured to control the power converter, the first switch, and the second switch.
The controller is configured to output a control signal to each of the first switch and the second switch to open the switches, and execute abnormality determination of determining whether or not the second switch has an abnormality in accordance with a voltage of the grid independent operation terminal detected by the voltage detector in a control state of causing the power converter to output a predetermined voltage.